1. Field of the Invention
The invention relates to an ink droplet jet device wherein partitions made of piezoelectric elements and constituting ink chambers are deformed to apply pressure to the ink therein so that ink droplets are jetted out through nozzles corresponding to the chambers.
2. Description of Related Art
Among many non-impact printers that are today replacing conventional impact printers, the ink jet printer is drawing attention because of its simplest-ever principle of operation and its ability to print in multiple gradations and with colors. Of the variations of ink jet printer, the drop-on-demand type is a printer that jets out only the ink droplets to be actually printed. As such, the drop-on-demand type ink jet printer is getting widespread acceptance thanks to its high jetting efficiency and low running cost.
Typical variations of the drop-on-demand type ink jet printer include the Kyser type disclosed in U.S. Pat. No. 3,946,398 and the thermal jet type disclosed in U.S. Pat. No. 4,723,129. Both types have formidable problems to be overcome: the Kyser type is difficult to manufacture in a small size, while the thermal jet type requires the ink to be capable of withstanding the high temperature applied to it.
One solution to the above problems is the shear mode type ink jet printer disclosed in U.S. Pat. Nos. 4,879,568 and 4,887,100. As shown in FIG. 6, a typical shear mode type ink jet printer 1 comprises a piezoelectric ceramic plate 2, a cover plate 10, a nozzle plate 14 and a substrate 41.
The piezoelectric ceramic plate 2 has a plurality of grooves 3 cut therein by a diamond blade or the like. Partitions 6, that separate the grooves 3 laterally, are polarized in a direction indicated by an arrow 5. The grooves 3 all have an identical depth and are in parallel to one another. The shallower the grooves 3 get, the closer they are to one edge 15 of the piezoelectric ceramic plate 2. Near the edge 15 are formed shallow grooves 7. Inside each groove 3, metal electrodes 8 are formed by sputtering or like process over the upper half of the two side walls flanking the groove. The metal electrodes 8 thus furnished over the upper half of the side walls of the grooves 3 are connected electrically by metal electrodes 9 provided in the shallow grooves 7.
The cover plate 10 is composed of a ceramic or plastic resin material. In the cover plate 10 an ink inlet 16 and a manifold 18 are formed by cutting or by grinding. The piezoelectric ceramic plate 2 has the upper surface of the side walls of the grooves 3 bonded by epoxy adhesive 20 (as shown in FIG. 8) to the side of the cover plate 10 where the manifold 18 is formed. With the tops of the grooves 3 thus covered, the ink jet printer 1 has a plurality of ink channels or ink chambers 4 (as shown in FIG. 8) formed laterally at equal intervals. As depicted in FIG. 8, the ink chambers 4 are each of a long, narrow shape having a rectangular cross section. All ink chambers 4 are filled with ink.
As illustrated in FIG. 6, a nozzle plate 14 is attached to the front edge of the piezoelectric ceramic plate 2 and that of the cover plate 10. The nozzle plate 14 has nozzles 12 provided thereon so as to correspond to the positions of the respective ink chambers 4. The nozzle plate 14 is made of such plastic resins as polyalkylene (e.g., ethylene) terephthalate, polyimide, polyether imide, polyether ketone, polyether sulfone, polycarbonate, and cellulose acetate.
The substrate 41 is bonded by an epoxy adhesive or the like to the surface of the piezoelectric ceramic plate 2 which is opposite to the side where the grooves 3 are formed. The substrate 41 has conductive layer patterns 42 formed thereon, the patterns corresponding to the positions of the respective ink chambers 4. The conductive layer patterns 42 and the metal electrodes 9 at the bottom of the shallow grooves 7 are bonded by the use of lead wires 43 furnished through a known wire bonding process.
The structure of a control unit of the ink jet printer 1 of FIG. 6 will now be described with reference to a control unit block diagram of FIG. 7. The conductive layer patterns 42 formed on the substrate 41 are connected individually to an LSI chip 61. Also connected to the LSI chip 61 are a clock line 52, a data line 53, a voltage line 54 and a ground line 55. In keeping with the continuous clock pulses fed from the clock line 52 and according to the data sent over the data line 53, the LSI chip 61 determines through which nozzle 12 an ink droplet is to be jetted. Having selected the nozzle 12, the LSI chip 61 applies a voltage V from the voltage line 54 to the conductive layer pattern 42 connected to the metal electrodes 8 inside the corresponding ink chamber 4 to be activated. The LSI chip 61 applies 0 volt of the ground line 55 to the conductive layer patterns 42 connected to the metal electrodes 8 corresponding to the ink chambers 4 that are left inactive.
An operation of the ink jet printer 1 will now be described with reference to FIGS. 8 and 9. Suppose that given certain data, the LSI chip 61 determines to jet an ink droplet from an ink chamber 4B. The positive driving voltage V is then applied to metal electrodes 8E and 8F, and metal electrodes 8D and 8G are grounded G. As shown in FIG. 9, a partition 6B develops a driving electric field in the direction of an arrow 13B while a partition 6C develops a driving electric field in the direction of an arrow 13C. Because the driving electric field directions 13B and 13C are perpendicular to a direction 5 of polarization, the so-called thickness shear mode effect develops and causes the partitions 6B and 6C to deform rapidly toward the interior of the ink chamber 4B. The deformation of the partitions reduces the volume of the ink chamber 4B to boost the ink pressure therein. The resulting pressure wave causes an ink droplet to be jetted from the nozzle 12 (shown in FIG. 6) corresponding to the ink chamber. 4B.
When the driving voltage V is stopped, the partitions 6B and 6C return to their initial positions (as shown in FIG. 8), thereby gradually reducing the ink pressure inside the ink chamber 4B. The drop in ink pressure causes ink to be supplied into the ink chamber 4B from an ink tank, not shown, via the ink inlet 16 and the manifold 18 (as shown in FIG. 6).
Generally, the efficiency of jetting ink from the ink chamber 4 is enhanced by the setup shown in FIG. 11. With the direction of polarization reversed (i.e., the direction of the arrow 71), a positive voltage is first applied to the partitions 6B and 6C so that they move from each other when deformed. When the application of the voltage is stopped, the partitions 6B and 6C return to their initial positions (FIG. 8) to jet out an ink droplet through the nozzle.
What follows is a description of the behavior of the pressure wave inside the ink chamber 4 when the above driving method is used for the ink jet operation. The description is made specifically with reference to timing charts of FIGS. 10(A) through 10(C) and to a cross-sectional view of the ink jet printer 1 in FIG. 11. A voltage pulse B (FIG. 10(A)) is first applied to the ink chamber 4 to cause an ink droplet to be jetted therefrom. (To apply a voltage V to an ink chamber 4 connotes to apply the voltage V to the electrodes provided to that chamber.) Because the partitions 6 are polarized in the direction of the arrow 71, the partitions 6B and 6C are at first deformed to move away from each other (FIG. 11). The resulting increase in volume of the ink chamber 4B reduces the pressure therein including that in the vicinity of the nozzle 12 (FIG. 10(B)). This state is maintained for a period of L/a shown in FIG. 10(B). During that time, ink is sucked into the chamber through the manifold 18 (FIG. 6).
The period L/a is the time required for the pressure wave inside the ink chamber 4 to propagate one way in the longitudinal direction (from the manifold 18 to the nozzle plate 14 or in the opposite direction). The period L/a is thus determined by the length L of the ink chamber 4 and by the sonic velocity a within the ink.
According to the theory of pressure wave propagation, the pressure inside the ink chamber 4B is reversed exactly L/a after the leading edge of the pulse B; the pressure then shifts from negative to positive in nature. At that point in time, the voltage being applied to the ink chamber 4B reverts to 0 (FIG. 10(A)). This allows the partitions 6B and 6C to return to their initial positions, exerting pressure on the ink. The positive pressure adds up to another pressure generated by the partitions 6B and 6C returning to their initial positions. The result is a relatively high level of pressure (FIG. 10(B)) being applied to the ink inside the ink chamber 4B, whereby an ink droplet is jetted from the nozzle 12.
Where image information is to be presented graphically on a storage medium by the ink jet printer 1 described above, the structure of the printer makes it obvious that adjacent ink chambers 4 cannot jet ink droplets simultaneously. One solution to this bottleneck is the scheme disclosed in U.S. Pat. No. 5,016,028 wherein the ink chambers 4 are grouped into odd-numbered and even-numbered chambers, the two groups being alternated in the ink jet operation. Where there occurs a significant interference (i.e., cross talk) between ink chambers 4, it is proposed that the ink chambers 4 be divided into three or more groups overlapping with one another so that they may be driven in a rotational manner. For example, the ink chambers in the example of FIG. 9 are divided into three groups, chambers 4A and 4D belong to one group, chambers 4B and 4E to another group, and chambers 4C and 4F to a further group.
As shown in FIG. 12, Japanese Laid-Open Patent Publication No. 4-284253 discloses an ink jet printer 90 having electrodes 91A, 91B and 91C formed over each partition 6 and divided in the longitudinal direction thereof. In operation, ink is jetted from the ink chamber by applying a suitable voltage sequentially to the electrodes 91C, 91B and 91A, in that order, to vary the timing at which the partition 6 is deformed in its longitudinal direction.
One disadvantage of the scheme disclosed in U.S. Pat. No. 5,016,028 is as follows: suppose that where the ink chambers are divided into two groups, the ink chamber 4B receives a voltage to have its partitions 6B and 6C deformed. In this setup, the partitions 6B and 6C also act as a partition of the ink chamber 4A and that of the ink chamber 4C, respectively. The partitions 6B and 6C, when deformed to generate a pressure wave in the ink chamber 4B, thus generate pressure waves in the adjacent ink chambers 4A and 4C (FIG. 10(C)) as well. These pressure waves propagate in the ink within the ink chambers 4, are reflected by their side walls, and gradually attenuate while reciprocating inside the chambers 4. Clearly, the fluctuation of the pressure wave in the target ink chamber 4B from which to jet ink and the fluctuation of the pressure wave in the adjacent ink chamber 4C are always opposite in phase and proportional in amplitude. The pressure near the nozzles 12 of the ink chambers 4A and 4C is at first positive in nature at the leading edge of the pulse B and becomes a negative pressure PC upon elapse of L/a; the pressure thereafter reciprocates between positive and negative at intervals of L/a.
Suppose that a high driving voltage pulse B is applied to heighten the pressure PB. inside the ink chamber 4B in order to increase the ink jet velocity. In that case, the negative pressure PC in the adjacent ink chamber 4C increases correspondingly. The boosted negative pressure PC can destroy the meniscus of the nozzle 12, causing air to be introduced through the nozzle 12 into the ink chamber 4C wherein the air bubbles often prevent an ink droplet from being jetted at some future time. In addition, the negative pressure PC occurring for the period of L/a can also cause the meniscus to shift from the nozzle 12 into the ink chamber 4C. This often lets air go through the nozzle 12 into the ink chamber 4C, thereby hampering the normal ink jet operation.
The ink jet printer 90 disclosed in Japanese Patent Laid-Open No. 284253/1992 is complicated in structure because of the divided electrodes 91A, 91B and 91C. The major difficulty is that it takes time to form the three electrodes 91A, 91B and 91C within the narrow groove 3.